The present invention relates to a compact multiplexer/demultiplexer.
The so named multiplex method is a method for the common transmission of several independent signals (primary signals) via a single transmission medium. In a multiplexer, the different primary signals are combined to form a single multiplexed signal and transmitted. They are separated again in a demultiplexer on the receiver side. With the so named frequency multiplex method each signal occupies a frequency band of specific breadth. The basebands of several primary signals are thus shifted into higher frequency positions by modulation with graduated carrier frequencies, such that they end up alongside one another on the frequency scale. A frequency multiplexed signal thus forms which is then optionally amplified and transmitted. On the receiver side, the individual signals are as a rule separated from one another again with the help of frequency filters and restored to the original frequency position by demodulation.
To transmit signals on optical waveguides, the so named wavelength multiplex method is generally used which represents an optical frequency multiplex method. Light signals of different frequency are used for transmission in the multiplex method. In this case, each frequency used provides its own transmission channel on which the actual data to be transmitted can be modulated (amplitude modulation). The data signals modulated in this way are then brought together by means of corresponding optical coupling elements and transmitted simultaneously, but independently of one another. At the receiver of this optical multiplex connection the individual optical transmission channels are then reseparated in a demultiplexer with the help of corresponding wavelength-selective elements, e.g. passive optical filters, and converted into electrical signals with corresponding detector elements.
Optical multiplexers and demultiplexers have been known for a long time. Basically a multiplexer can also be used as demultiplexer by reversing the beam path and vice versa. In this case, instead of detectors which convert the received transmitted optical signals into electrical signals, lasers which produce the corresponding light signals to be transmitted need merely be used.
In the following, the description refers explicitly to demultiplexers. However, it is understood that the described features can also be used advantageously in multiplexers, wherein the beam direction simply reverses.
Demultiplexers generally have an input connection for inputting an optical signal which has signal components of different wavelengths, at least one wavelength-sensitive element as well as at least two focussing elements, wherein the wavelength-sensitive element and the focussing elements are arranged such that at least one part of an optical signal input via the input connection first strikes the wavelength-sensitive element and then a focussing element, and a further part first strikes the wavelength-sensitive element and then a different focussing element. By wavelength-selective element is meant any element which, when placed in the beam path, influences one, several or indeed all wavelength channels. By influencing is meant for example reflecting, absorbing, amplifying, attenuating, interrupting or polarizing.
By focussing element is meant any element which can bring incident parallel light beams together substantially at one point, the so named focal point or focus. For example, optical lenses or concave mirrors can be used as focussing elements.
In the simplest case the demultiplexer has only one wavelength-sensitive element and two focussing elements. An input signal which consists of two separate frequency components (frequency channels) is then directed onto the wavelength-sensitive element which reflects one frequency component and allows transmission of the other. The focussing elements are arranged such that one receives the transmitted beam and the other the reflected beam, and directs them to the respective focal point. If a suitable radiation detector, e.g. a photodiode, is arranged at the corresponding focal points, the amplitude, i.e. the radiation intensity of the frequency signal, can be recorded electrically. Generally, however, a demultiplexer has a plurality of wavelength-sensitive elements onto which the signal is successively directed along the beam path, wherein each wavelength-sensitive element separates a wavelength channel from the rest of the signal. The arrangement of several wavelength-sensitive elements is also called the filter cascade.
The production of demultiplexers is however generally very costly. This is due i.a. to the required adjustment. The combined signal from a corresponding transmission medium, e.g. a glass fibre, must be directed onto corresponding detector elements with the help of a precisely adjusted arrangement of filters and mirrors in order to bring about an effective splitting of the signal into its individual channel components. Known demultiplexers also have comparatively large dimensions.
An optical wavelength demultiplexer comprising an optically transparent structure is already known from EP 1 004 907. The signal emerging from a glass fibre is guided inside the transparent material. The transparent structure is designed in two parts, wherein corresponding optical filters are arranged between the two parts. Although this demultiplexer is already compact, it can be produced only at great expense in terms of production engineering and must be adjusted in a complex manner. Therefore it has already been proposed in DE 10 2005 010 557 to arrange a shaped part having several focussing elements as well as a carrier plate having several wavelength-sensitive elements on a mounting plate. The cost of adjusting the focussing elements relative to the carrier plate has thereby been simplified. However, in the embodiment described in DE 10 2005 010 557 the adjustment of the detectors is very complex.